Field of the Invention
This invention is directed to a magnesium electrochemical cell containing a negative electrode having magnesium metal that is formed on a current collector by electrochemical deposition of a magnesium salt electrolyte upon charging the cell. The invention is further directed to a rechargeable magnesium battery constructed by a process wherein an active magnesium anode is prepared by electrodeposition of magnesium metal from a dissolved magnesium electrolyte salt onto a current collector.
Discussion of the Background
Lithium ion batteries have been in commercial use since 1991 and have been conventionally used as power sources for portable electronic devices. The technology associated with the construction and composition of the lithium ion battery (LIB) has been the subject of investigation and improvement and has matured to an extent where a state of art LIB battery is reported to have up to 700 Wh/L of energy density. However, even the most advanced LIB technology is not considered to be viable as a power source capable to meet the demands for a commercial electric vehicle (EV) in the future. For example, for a 300 mile range EV to have a power train equivalent to current conventional internal combustion engine vehicles, an EV battery pack having an energy density of approximately 2000 Wh/L is required. As this energy density is close to the theoretical limit of a lithium ion active material, technologies which can offer battery systems of higher energy density are under investigation.
Magnesium as a multivalent ion is an attractive alternate electrode material to lithium, which can potentially provide very high volumetric energy density. It has a highly negative standard potential of −2.375V vs. RHE, a low equivalent weight of 12.15 g/mole of electrons and a high melting point of 649° C. Compared to lithium, it is easy to handle, machine and dispose. Because of its greater relative abundance, it is lower in cost as a raw material than lithium and magnesium compounds are generally of lower toxicity than lithium compounds. All of these properties coupled with magnesium's reduced sensitivity to air and moisture compared to lithium, combine to make magnesium an attractive alternative to lithium as an anode material.
Magnesium (Mg) batteries are being researched as a candidate for post lithium-ion systems. They are expected to be high energy battery systems, due to the high volumetric capacity made available via the two electron transfer per Mg. Much effort has been devoted to electrolytes and cathodes in order to maximize the advantage of Mg metal as anode and to establish the entire Mg metal battery system. In the electrolyte research, systems showing high oxidation potential, less corrosion and good compatibility with Mg metal have been reported. In this regard, some Grignard-based electrolytes and borohydride-based electrolytes work well as Mg battery electrolyte, while typical organic solvents do not. Additionally, there is much ongoing research and development directed to cathode active materials and the construction of new cathodes showing increasingly higher rechargeability and higher durability at practical rates. Chevrel phase Mo6S8 has been demonstrated to be a cathode material of good performance with a magnesium anode. Other materials showing potential as cathode actives for a magnesium battery include elemental sulfur, manganese dioxide and vanadium oxide.
However, an ongoing problem with a magnesium anode and construction of a magnesium electrochemical cell is associated with recognition that the magnesium ion is doubly charged and therefore, does not pass through films formed on the electrode surface when magnesium chemically interacts with the electrolyte. Such films impose high impedance to the system, in the range greater than ten thousand ohms and as a result the charge-discharge voltage quickly hits upper and lower cut-off limits, resulting in almost no capacity and no voltage plateau. FIGS. 4 and 5 demonstrate such performance. Thus, magnesium batteries have been prepared under conditions and methods designed to prevent or control this problem. However, until now the problem has remained and a method to prepare and assemble a magnesium electrochemical cell or battery has been needed.
The inventors are directing effort and resources to the study of all aspects of the production of a magnesium battery of sufficient capacity and cycle lifetime to be useful as a power source for utilities requiring a high capacity and high cycle lifetime and have recognized the importance of addressing and solving the problem associated with high impedance film formation in a magnesium battery.
Therefore, an object of the present invention is to provide a method to prepare a magnesium electrochemical cell having low or no impedance due to film formation on the magnesium anode during the construction process.
Another object of the present invention is to provide a rechargeable magnesium battery having high capacity and high cycle lifetime.